Sunday, 6 November 2011

Preliminary Evaluation of a Proposed “Younger Dryas Impact” Crater

Preliminary Evaluation of a Proposed “Younger Dryas Impact” Crater

Using Google Earth terrain maps, Mr. Thomas Lee Elifritz found
what he speculates might be impact crater of the “12,900 BP
Younger Dryas Clovis cometary ice sheet impact” (Anonymous
2011, Elifritz 2009a, 2009b). His interpretation is based solely
upon a vague circular feature, which is centered around
49˚ 10’ 41” N., 88˚ 43’ 9” W. and is about 25 to 26 km in
diameter. The edge of this feature lies at the southern end of
Black Sturgeon Lake, along cliffs just north and east of Nonwaith
Lake, along a well-defined ridge, Fox Mountain, just east and
north of Disraeli Lake, and through middle of Black Mountain
Lake.

As noted by Hart and Magyars (2004), this area was mapped
by Coates (1972) at a scale of 1:63 360. This geologic map
included an interpreted geological section. Later, Sutcliffe (1982)
mapped the geology of the northeast part of this area from
immediately south of Lake Nipigon to the southern edge of
Black Sturgeon Lake at a scale of 1:50 000. Also, various
studies, i.e. Sutcliffe (1987), Hart and McDonald (2007),
Heaman et al. (2007), and Hollings et al. (2007) have
investigated in detail the areal intrusive rocks, Nipigon
diabase sills and ultramafic intrusions of the Disraeli and
Seagull–Lecky lakes areas. The sedimentary rocks of the
Sibley Group, which underlie this area have also been studied
by various investigators, i.e. Franklin et al. (1980), Chile
(1986), Regale (2003), and Frolic and Zanies (2011). Recent
geologic mapping, reports, and talks, Hart and Magyars
(2004), Magyars et al. (2004), Hart 2005a, 2005b), and Hart
and Olson (2005) provide very detailed discussion, geologic
maps, and subsurface cross-sections of the northern Black
Sturgeon River–Disraeli Lake region based upon detailed
field work conducted during the summer of 2003 and existing
subsurface data. Finally, frequent mineral exploration and
prospecting has occurred within this area. This exploration has
created a collection of lithologic logs of and actual cores from
numerous drill holes from which cross-sections of the subsurface
geology of this area have been created by Coates (1972),
Hart and Magyars (2004), Magyars et al. (2005), Hart
and Thompson (2007) and others.

Both sedimentary rocks of the Sibley Group and the Nipigon
Diabase Sills exhibit a lack of any deformation that can be
associated with the entire circular feature hypothesized
to be an impact crater. These strata are flat-lying to slightly
dipping. They are disturbed only by regional north and
northwest-trending faults that are interpreted to have locally
created a block-faulted asymmetric basin or graben. These
faults are considered the result of the reactivation of earlier
Archean structures during the Late Proterozoic (Coates,
1972; Franklin et al., 1980; Hart and Magyars, 2004;
Magyars et al., 2005).

Only along the northeast rim is faulting even roughly
associated with the rim of this feature. The rim is not the
direct result of fault movement. Rather, its is the result
of differential erosion of less resistant sedimentary rocks
of the Sibley Group and more resistant metasedimentary
and tonalite intrusives of the Quetico Subprovince on either
side of faults. In addition, the faults are high angle faults
with orientations and dips typical of regional faulting
(Coates, 1972; Franklin et al., 1980; Hart and Magyars,
2004; Magyars et al., 2005). These faults lack the
characteristics of those exhibited by extraterrestrial
impact structures (Melosh 1989).

Furthermore, There is a complete lack of any evidence for
any recognizable structural control of the western and
southern edge of this feature, which is composed of roughly
arcuate ridges, Fox, Wolf, and Eagle mountains. The
geologic mapping and cross-sections of Coates (1972),
Hart 2005a, Hart and Magyars (2004), Magyars et al.
(2005), and Hart and Thompson (2007) show that they
consist of relatively flat-lying and undeformed sedimentary
rocks of sedimentary rocks of the Sibley Group and
interlayered sills of the Nipigon Diabase. There is a
complete absence of any deformation that could be
associated with an extraterrestrial impact (Melosh 1989).
If there is structural control to these ridges, it is the
result something considerably more subtle and cryptic
than an extraterrestrial impact.

Concerning the circular feature, which Elifritz (2009a,
2009b) proposed to be a possible Younger Dryas Impact,
there exists sufficient information to make a preliminary
evaluation of this hypothesis. The undeformed and
relatively flat-lying nature of the sedimentary strata and
interlayered diabase sills readily refutes any idea that this
feature is an impact structure any sort or age given the
25-km diameter of this feature. An extraterrestrial origin
of this feature is further refuted by the lack of any
significant consistent relationship between regional
faults and the feature and the age of the faulting.
Despite its roughly circular shape, it appears that this
circular feature is combined result of past differential
weathering and glacial and glacial and meltwater
erosion of the Proterozoic sills and Sibley Group
metasedimentary strata that has been influenced by
Proterozoic faulting and possible cryptic structural
controls.

If anything, this feature is most likely a example of the
problems with relying entirely on terrain maps (digital
elevation models) as a basis for identifying extraterrestrial
impact crates. It shows that existing geological research
also needs to be consulted before proposing any circular
feature as an impact crater. This feature is likely a
classic example of Riemold (2007)’s “Impact Crater
Bandwagon.”

Dare to Add venture Fur Ther???

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